Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A

TL;DR

This paper analyzes multiwavelength data of GRB 070419A, revealing a long-lived central engine with energy injection that explains complex optical and X-ray light curves, challenging standard shock models.

Contribution

It demonstrates the necessity of a finely tuned, long-lasting central engine to account for the observed optical and X-ray emission features in GRB 070419A.

Findings

Early emission explained by a central engine active for at least 250 s.

Optical light curve complexity requires a reverse shock component.

Standard shock models are insufficient; a long-lived central engine is needed.

Abstract

We present a comprehensive multiwavelength temporal and spectral analysis of the FRED GRB 070419A. The early-time emission in the $\gamma$-ray and X-ray bands can be explained by a central engine active for at least 250 s, while at late times the X-ray light curve displays a simple power-law decay. In contrast, the observed behaviour in the optical band is complex (from 10$^2$ up to 10$^6$ s). We investigate the light curve behaviour in the context of the standard forward/reverse shock model; associating the peak in the optical light curve at $\sim$450 s with the fireball deceleration time results in a Lorenz factor $\Gamma \approx 350$ at this time. In contrast, the shallow optical decay between 450 and 1500 s remains problematic, requiring a reverse shock component whose typical frequency is above the optical band at the optical peak time for it to be explained within the standard model. This predicts an increasing flux density for the forward shock component until t $\sim$ 4 $\times$ 10$^6$ s, inconsistent with the observed decay of the optical emission from t $\sim$ 10$^4$ s. A highly magnetized fireball is also ruled out due to unrealistic microphysic parameters and predicted light curve behaviour that is not observed. We conclude that a long-lived central engine with a finely tuned energy injection rate and a sudden cessation of the injection is required to create the observed light curves - consistent with the same conditions that are invoked to explain the plateau phase of canonical X-ray light curves of GRBs.